Commercial and domestic refrigerators and freezers are provided with a refrigeration unit for cooling. The refrigeration unit typically has a compressor driven by a compressor motor, a condenser and an evaporator. As the refrigeration unit operates, water vapor condenses on the evaporator and results in the build-up of frost and ice on the evaporator. The build-up of frost and ice on the evaporator results in diminished airflow through the evaporator and a reduction in the ability of the refrigeration unit to cool the air within the refrigerator or freezer. Furthermore, the buildup of frost and ice on the evaporator reduces the rate of heat transfer between the evaporator and the air. To enhance the efficiency of refrigerators and lower their power consumption, many refrigerators are designed to periodically defrost the evaporator. Defrost devices, such as heaters, are often used to hasten the defrost operation. Also known are refrigerators that defrost on demand by computing a parameter indicative of an accumulation of ice and, in response, initiate a defrost operation.
In order to reduce the cost of operation per unit time, it is important to judiciously choose the time of defrost. In the event that the refrigerated device is defrosted more often than necessary, the cost of unnecessary defrosting is incurred. Conversely, defrosting less often than warranted leads to an accumulation of too much frost on the evaporator, which concomitantly reduces the efficiency of any compressor cycle, leading to excessive compressor runtimes, and raising the operating cost of the refrigeration device.
There is an ongoing need for methods and systems for controlling defrost cycles of refrigeration units. Current techniques have, unfortunately, produced unsatisfactory solutions in many situations, leading to excessive operating costs of refrigeration devices.
For example, techniques that rely on extrapolating historical compressor cycle and/or defrost data to the present and future can produce inaccurate solutions when the historical data is non-recurring.
The cumulative time method involves monitoring of the cumulative time a compressor is run during a cooling interval. The interval between defrost cycles is then varied based on the cumulative time the compressor is run and/or the elapse time since a previous defrost. Unfortunately, the cumulative time method fails in environments where there is little or no need to run a defrost cycle, such as low humidity environments or environments where there is little or no door opening. In these situations, there is little or no frost accumulation on the evaporator, yet the compressor continues to run, and the refrigeration device executes unnecessary defrost cycle, leading to an unnecessarily elevated cost of operation.
Other techniques analyze and compare the length of one or more defrost cycles or runtimes. Thus, compressor cycles with an increased compressor runtime, according to some of these techniques, are considered indicative of a less efficient compressor cycle due to frost buildup on the evaporator. Thus, according to these techniques, these longer individual compressor runtimes can trigger a defrost cycle. Unfortunately, longer individual compressor runtimes can be indicative of a number of situations, including a need to defrost as well as other factors, such as a door to the device being open for an unusually long time and/or placement of a warm product within the device. Although certain patches have been suggested to this solution, such as explicitly measuring time that the door is open and incorporating into the defrost algorithm, the basic problem of using erroneous defrost indications remains.
Below is a list of U.S. Patents and US published patent applications providing potentially relevant background art. Each of these U.S. Patents and US published patent applications are incorporated herein by reference. U.S. Pat. No. 5,816,054; U.S. Pat. No. 5,493,867; U.S. Pat. No. 5,440,893; U.S. Pat. No. 6,668,566; US 20020088238; US20030084672.
There is an ongoing need for methods and systems for controlling defrost cycles of refrigeration units. Preferably, these methods and systems would include a correction mechanism for appropriately treating potentially erroneous defrost indications such as long compressor runtimes due to placing of warm foods or beverages within the refrigerator unit.